THURSDAY, Oct. 22, 2015 (HealthDay News) -- Scientific debate continues to simmer over the so-called "obesity paradox" -- the apparent ability of obese or overweight people to better withstand chronic illnesses, such as diabetes or heart disease, and outlive thinner people with those same maladies.

But the obesity paradox is built on a foundation of shaky research, a new study contends. Earlier studies failed to account for two important factors that negate the supposedly protective effects of obesity: weight history and smoking, the researchers said.

People who are seriously ill tend to lose weight as they near death, the study authors explained. And that fact influences the data enough to create the false perception of an obesity paradox, argued lead author Samuel Preston, a professor of sociology at the University of Pennsylvania in Philadelphia.

Previous studies also have failed to account for smoking, which is incredibly unhealthy but also tends to ramp up a person's metabolism and keep them slim, Preston added. Smokers are less likely to be obese, and those who are obese are less likely to smoke, the study authors said.

"When we adjust for those biases, we reverse the obesity paradox in people with cardiovascular disease," he concluded. "The obesity paradox exists, but the cause of it is not the benefit of being overweight. The principal causes of the paradox are these two biases."

For the study, researchers examined data from more than 30,000 people participating in the U.S. National Health and Nutrition Examination Survey between 1988 and 2011. The study focused on the nearly 3,400 who had heart disease.

Most studies that have looked at the possibility of an obesity paradox only included a person's weight when they are surveyed, Preston said.

For example, a 300-pound man might have dropped a third of his weight within the last month due to illness, but the study would count him as always weighing 200 pounds.

This time, the research team opted to include weight history. That way, they could include the health risks of long-term obesity in their analysis, even if someone had recently lost weight, Preston explained.

Adding weight history allowed them to identify a control group of people who suffered from heart disease but maintained a normal weight their whole lives. These people tended to live longer than people who were overweight or obese for most of their lives, essentially causing the obesity paradox to disappear, the researchers said.

The same thing happened when researchers factored in smoking. The risk of death during the study period was about 50 percent higher in people who were overweight or obese, compared with normal-weight adults, when researchers only looked at those who'd never smoked.

The study authors hope their findings could improve disease treatment because some clinicians may use the obesity paradox in patient care decisions.

"There's every reason to imagine that clinicians are at least confused," Preston said, "and in some cases, are believing that being overweight or obese is a good thing among people with cardiovascular disease, diabetes and other conditions for which a paradox has been demonstrated."

Dr. Suzanne Steinbaum, director of Women's Heart Health at Lenox Hill Hospital in New York City, said these results show that the obesity paradox is "probably just a statistical mistake."

"What this really demonstrates is that statistics can be an interesting thing," Steinbaum said. "If you don't analyze the data in an appropriate way, you may come to an incorrect conclusion."

However, it may be too soon to completely discount the obesity paradox, said Dr. Robert Eckel, a professor and chair in atherosclerosis at the University of Colorado School of Medicine in Aurora.

"This raises an important issue, in terms of further analysis of this controversy," Eckel said of the new study.

But Eckel added that while the study focused on people with heart disease, it appears to count deaths from any cause rather than deaths caused specifically by heart problems. That could undermine the findings, he suggested.

"It would be of interest to know if the people with cardiovascular disease were more likely to die of cardiovascular disease than any other cause," he said.